Ultrasonic waves have characteristics that electromagnetic waves such as a laser beam and a microwave never provide in that the former deeply propagates inside a living-body at short wavelengths and can be focused on an arbitrary point. Research and development of ultrasonic therapy which make good use of these characteristics have been actively advanced. The effects of ultrasonic waves on a living body, which can be made use of at the time of treatment, are roughly classified into thermal effects and sonochemical effects. The former thermal effects are produced when tissue absorbs an ultrasonic wave and consequently heat is generated. The treatments that medically use the thermal effects are roughly classified into “hyperthermia” and “heat coagulation therapy”. Hyperthermia involves continuously heating an affected part to about 40 to 50° C. for treating tumors or the like. Heat coagulation therapy involves using high intensity focused ultrasound (HIFU) to heat a minute region of an affected part in a short period of time up to a temperature of, e.g., 70 to 100° C., which can cause tissue denaturation.
“Hyperthermia” used for treating tumors is a therapy that makes use of the characteristic that tumor cells are weak in persistent high temperature (about 43° C.) as compared with normal cells. Although slowing the growth of a tumor, however, hyperthermia does not reach the satisfactory level for treatment because of the following disadvantages: The capability of directly and drastically necrotizing tumor cells is low. Since the increase in temperature of an affected part is controlled by the blood flow of the tissue surrounding the affected part and heat conduction, it is not easy to keep temperature required for treatment unchanged. In addition, since a temperature-increasing region is not sufficiently localized, the balance between curative effects and stress (side effect) on a living body is not satisfactorily achieved. The hyperthermia, therefore, is often used as combined therapy in combination with radiation therapy in the actual clinical field.
On the other hand, heat coagulation therapy using HIFU involves focusing a high intensity ultrasonic wave on a minute region having a size of the order of millimeters so as to increase the temperature to a level at which tissue denaturation is immediately caused. The heat coagulation therapy using HIFU is different from hyperthermia described above in temperature increasing at a target region to be treated, and in a change in tissue caused by the increased temperature. Heat generated in tissue is carried away by heat conduction and the blood flow. In the case of heat coagulation therapy, however, a high intensity ultrasonic wave increases the temperature in a focused part to a temperature level of the protein coagulation or more to cause the coagulation in a period of time that is much shorter than a period of time (about 1 minute) required until the heat transport and the heat generation caused by the ultrasonic wave enter an equilibrium state. Since the ultrasonic wave density is low in regions other than a region corresponding to a focus, the temperature in these regions does not reach the temperature of heat denaturation. As a result, the tissue denaturation occurs only in a region close to the focus. At present, the therapy using HIFU is applied to therapy for prostatic hyperplasia, prostatic cancer, and uterine fibroid.